The National Stuttering Association - westutter.org - has resources that are worth looking into. It would have helped me if my parents had simply acknowledged my stuttering and been part of my process to develop techniques to deal with it. Knowing that he can vent to you about it and that you are with him for his ride through stuttering should help him. Usually you can’t eliminate it but you can prevent it from being disabling and you can learn to deal with the negative feelings that can arise from it.